In recent years, infrared cut filters transmitting light in a visible wavelength range (420 to 630 nm), but cutting light in a near-infrared wavelength range (700 to 1100 nm) have been used in various applications.
For example, in imaging apparatuses such as a digital still camera, a digital video, a cellular phone camera using solid-state imaging devices such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor) image sensor, and display apparatuses such as an automatic exposure meter using a light-receiving element, there is conventionally a spectral sensitivity from the visible wavelength range to the near-infrared wavelength range around 1100 nm. Accordingly, the infrared cut filter as stated above is disposed between an imaging lens and the solid-state imaging device or the light-receiving element to correct visibility so as to approximate the sensitivity of the solid-state imaging device or the light-receiving element to the visibility of a person to obtain good color reproducibility.
As the infrared cut filter as stated above, for example, a filter is known made up of a glass where CuO or the like is added to a fluorophosphate-based glass and a phosphate-based glass so as to selectively absorb the light in the near-infrared wavelength range. This glass filter contains CuO with a lot of P2O5 as an essential component, and exhibits blue-green by forming Cu2+ ion coordinated at a lot of oxygen ions in an oxidizing melting atmosphere, and exerts a near-infrared absorption characteristic.
However, an absorption ability in the near-infrared wavelength range of this glass filter is lowered as it goes to a long-wavelength side. It is possible to increase the absorption ability in the near-infrared wavelength range by increasing an addition amount of CuO, but a Cu ion concentration which has absorption in a visible short-wavelength region is simultaneously increased, and therefore, a transmittance of light in a visible wavelength range is lowered.
Accordingly, practically, an optical interference multilayer film in which two or more kinds of dielectric thin films having a refractive index difference which are transparent or have an absorption in an infrared wavelength range are alternately stacked at one surface or both surfaces of a substrate of the above-stated glass is formed to thereby compensate the near-infrared cutting ability of the glass to be used as an infrared cut filter having a cutting property up to around 1100 nm.
On the other hand, in recent years, in a solid-state imaging device, a pixel number becomes larger, and sensitivity becomes higher, and one capable of receiving light in a long-wavelength range over 1200 nm becomes popular. Namely, in recent years, a small-sized and high resolution solid-state imaging device has been required, and there is a tendency to increase in number of pixel number large without enlarging a light-receiving area. As a result, resulting from decrease in an absolute amount of incident light in accordance with an area reduction in a unit pixel size, an electron number per each pixel to be an original of an output signal decreases, sensor sensitivity (a current output in accordance with light intensity at a semiconductor layer) decreases, and it becomes essential to make the sensitivity per pixel high. Therefore, it is general to increase an absorption amount by thickening the semiconductor layer, but electromagnetic wave absorption coefficients by the semiconductor layer are smaller in a long-wavelength component compared to a short-wavelength component, and therefore, the long-wavelength component reaches a semiconductor deep layer part. Accordingly, one capable of receiving the light having the longer wavelength such as over 1200 nm compared to a conventional one increases.
Besides, a mechanism to cut the light in the near-infrared wavelength range by the dielectric multilayer film is owing to a reflection action of light resulting from an interference of substances having the refractive index difference as stated above. Therefore, it enables a steep cutoff characteristic. On the other hand, the light in the near-infrared range which is incident on the dielectric multilayer film is reflected by the dielectric multilayer film, but becomes stray light in the imaging/display apparatus including the solid-state imaging device without being attenuated. This stray light is obliquely incident on the dielectric multilayer film again, and thereby, there is a possibility in which the stray light reaches the solid-state imaging device to be recognized as noises without being fully cut by the dielectric multilayer film.
Further, a digital still camera embedded in a recent smartphone has been reduced in thickness, and an optical path length from a previous stage lens to an imaging apparatus becomes short. As a result, there is a tendency in which the light which is obliquely incident on the infrared cut filter increases more and more. Therefore, a reflection and light-shielding characteristic by the dielectric multilayer film is further wavelength-shifted, and the light-shielding characteristic at a band from 1100 nm to over 1200 nm becomes insufficient.
Accordingly, in the infrared cut filter, improvement in the cutting performance at the band from 1100 nm to over 1200 nm has been required. For example, it is conceivable to increase a thickness of a glass filter to enable this object. However, in this case, a device thickness increases, and it is impossible to meet the demand or the like for reduction in thickness of the digital still camera. Besides, it is possible to elongate the cut wavelength range by the dielectric multilayer film up to the long-wavelength range over 1200 nm, but it is necessary to increase the number of stacks and a total film thickness. As a result, the number of processes increases, in addition, there is a problem of foreign substances attached to a film at a film forming time, and other problems such as lowering of production yield and increase in manufacturing cost are incurred.